This invention relates to an efficacious and safe method of controlling rodent population. Rodents, especially Norway rats common to the United States, roof rats, black rats, ground squirrels, prairie dogs, mountain beaver, etc., feed on a wide variety of foods. Rats, in particular, do a tremendous amount of damage by their eating habits, destroying agricultural crops, stored grain and stored produce, as well as contributing to the spread of disease through their infestation of urban areas. Attempts to control or eliminate rodents have been made for many years. Because the rodents quickly learn to avoid the poisons and poison impregnated foods, the use of poisoned food or baits as practiced to date does not provide a long term solution to the rodent problem.
The need for economical rodent control methods for agricultural, commensal and industrial uses, necessitates a material that can be easily dispersed in the fields or in buildings and provide an efficacious long term method of rodent control. Many of the baits depend upon the impregnation of seeds, fruits, peanut butter, etc. with a poison. The rodents quickly learn to avoid such poison bait by the odor or taste, particularly when the discomfort of the ingestion of a sub-lethal quantity is associated with the bait. Therefore, it is advantageous to prepare a poison that has its odor and taste effectively masked and the onset of the reaction to the poison is delayed until a lethal amount is consumed and the rodent has left the bait area.
Attempts to encapsulate rodenticides have been made but generally enough rodenticide is entrapped in the encapsulating material or will permeate through it to allow the rodent to identify the fact that a poison is present. Also, methods have been developed to prepare enteric coated rodenticides. See, for example, Shuyler U.S. Pat. No. 2,957,804 wherein a rodenticide, such as zinc phosphide, is surrounded by a resin material that will dissolve in the alkaline condition of the rodent's intestines rather than the acidic conditions of the rodent's stomach. The theory is that the rodent that has eaten the poison would not realize that it was deadly since there would be considerable time delay from the ingestation to death, and thus would not avoid the enteric coated material. In order to accomplish this result, Shuyler utilizes a relatively thick coating wherein the coating is essentially present in an amount of at least about 50% of the weight of the rodenticide. The efficacy of such encapsulated material has been shown to be no more than unencapsulated material because the amount of encapsulant needed to mask the material also decreases its availability as a poison. More importantly, the zinc phosphide does not react substantially in alkaline environments to produce phosphine.
Zinc phosphide is well known as a rodenticide and is especially effective when used against rats and field mice. Zinc phosphide is a dark grey crystalline material with a faint garlic-like odor and is a very stable compound when stored in a dry state. It is substantially insoluble in water, but reacts with acid to produce phosphine, a deadly poison. It is effective against many species of rodents. However, it suffers from the disadvantage that rodents easily identify it in a bait and avoid it. Zinc phosphide is considered one of the safest rodenticides to control rats and mice because of its emetic properties and the fairly rapid dissipation of the poison in the rodent's body. Thus, children, domestic animals, etc. will regurgitate the poison before it is lethal, whereas rodents cannot regurgitate. Zinc phosphide in the body of a dead rodent reacts with the fluids in the rodent's body and is consumed in a way which eliminates the hazard of secondary poisoning of other animals who may eat the dead rodent. For this reason zinc phosphide has been widely used for controlling rodents in agriculture so as to protect rice, sugarcane and other growing plants against attack. In the western United States, for example, zinc phosphide has had widespread use to control ground squirrels, and the like.
In co-pending U.S. application Ser. No. 256,742, it was discovered that when particles of zinc phosphide are coated with from about 2% to about 10% by weight of a thermoplastic polyamide, such encapsulation masks the odor and taste of the toxicant so as to avoid detection by the rodent, while at the same time increasing its efficacy as a rodenticide by allowing the phosphide to be released in the stomach of the rodent to generate a lethal dose of phosphine.
Although it has been demonstrated that encapsulated zinc phosphide can provide an effective rodenticide, further investigation and experimentation has suggested additional improvements to increase the efficacy of the rodenticide, primarily as a result of improvements to the binder and bait comprising the rodenticide, as well as by including additives in the formulation.
For instance, in formulating rodenticidal compositions, it is commonplace to use oils, such as corn oil, as a binder for adherence of the toxicant to the bait or food. Without a binder, the zinc phosphide would tend to sink to the bottom of the bait and not be eaten by the rodent. The use of corn oil as a binder is required in EPA formulations. However, it has been observed that corn oil actually decreases the efficacy of the toxicant by forming an additional coating on the encapsulated zinc phosphide preventing release of the zinc phosphide in the rodent's stomach.
Another problem has been encountered when zinc phosphide is added to certain bait compositions which contain grains or processed grains. It has been found that when the zinc phosphide is added to these bait compositions, phosphine is generated resulting in rejection of the bait by the rodent.
It is thus a primary object of the present invention to provide an improved rodenticide which substantially overcomes the problems heretofore associated with the use of zinc phosphide as a toxicant in formulated bait compositions.